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| author | Chris Cannam |
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| date | Wed, 20 Mar 2013 15:35:50 +0000 |
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| 1 <html lang="en"> | |
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| 3 <title>Dynamic Arrays in C - FFTW 3.3.3</title> | |
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| 16 Copyright (C) 2003 Matteo Frigo. | |
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| 18 Copyright (C) 2003 Massachusetts Institute of Technology. | |
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| 49 <a name="Dynamic-Arrays-in-C"></a> | |
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| 51 Next: <a rel="next" accesskey="n" href="Dynamic-Arrays-in-C_002dThe-Wrong-Way.html#Dynamic-Arrays-in-C_002dThe-Wrong-Way">Dynamic Arrays in C-The Wrong Way</a>, | |
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| 54 <hr> | |
| 55 </div> | |
| 56 | |
| 57 <h4 class="subsection">3.2.4 Dynamic Arrays in C</h4> | |
| 58 | |
| 59 <p>We recommend allocating most arrays dynamically, with | |
| 60 <code>fftw_malloc</code>. This isn't too hard to do, although it is not as | |
| 61 straightforward for multi-dimensional arrays as it is for | |
| 62 one-dimensional arrays. | |
| 63 | |
| 64 <p>Creating the array is simple: using a dynamic-allocation routine like | |
| 65 <code>fftw_malloc</code>, allocate an array big enough to store N | |
| 66 <code>fftw_complex</code> values (for a complex DFT), where N is the product | |
| 67 of the sizes of the array dimensions (i.e. the total number of complex | |
| 68 values in the array). For example, here is code to allocate a | |
| 69 5 × 12 × 27 rank-3 array: | |
| 70 <a name="index-fftw_005fmalloc-121"></a> | |
| 71 <pre class="example"> fftw_complex *an_array; | |
| 72 an_array = (fftw_complex*) fftw_malloc(5*12*27 * sizeof(fftw_complex)); | |
| 73 </pre> | |
| 74 <p>Accessing the array elements, however, is more tricky—you can't | |
| 75 simply use multiple applications of the ‘<samp><span class="samp">[]</span></samp>’ operator like you | |
| 76 could for fixed-size arrays. Instead, you have to explicitly compute | |
| 77 the offset into the array using the formula given earlier for | |
| 78 row-major arrays. For example, to reference the (i,j,k)-th | |
| 79 element of the array allocated above, you would use the expression | |
| 80 <code>an_array[k + 27 * (j + 12 * i)]</code>. | |
| 81 | |
| 82 <p>This pain can be alleviated somewhat by defining appropriate macros, | |
| 83 or, in C++, creating a class and overloading the ‘<samp><span class="samp">()</span></samp>’ operator. | |
| 84 The recent C99 standard provides a way to reinterpret the dynamic | |
| 85 array as a “variable-length” multi-dimensional array amenable to | |
| 86 ‘<samp><span class="samp">[]</span></samp>’, but this feature is not yet widely supported by compilers. | |
| 87 <a name="index-C99-122"></a><a name="index-C_002b_002b-123"></a> | |
| 88 <!-- =========> --> | |
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